The present disclosure relates to imaging members and, more specifically, to methods for making dispersions, which are of various rheologies, various pigment/binder ratios, various particle sizes, and possess less impurities or large particles. These dispersions, in turn, may be utilized to form layers of imaging members and photoreceptors.
Electrophotographic photoreceptors may be in the form of plates, rigid drums, flexible belts, and the like. Electrophotographic photoreceptors may be prepared with either a single layer configuration or a multilayer configuration. Multilayered photoreceptors may include a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generation layer, a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer (CGL) and the charge transport layer (CTL).
One technique for coating cylindrical or drum shaped photoreceptor substrates to form these layers, including charge generation layers, involves dipping the substrates in coating baths. For example, baths used for preparing charge generation layers may be prepared by dispersing photoconductive pigment particles in a solution containing a film forming binder. Newtonian dispersions may be utilized for dip coating since uniformity in the charge generation layer is more likely to occur. Methods for forming such Newtonian dispersions include those disclosed in U.S. Pat. No. 6,057,075, the disclosure of which is hereby incorporated by reference in its entirety, wherein a stable Newtonian coating dispersion may be formed by preparing a first stable Newtonian dispersion, and adding a polymer to said dispersion to form the stable Newtonian coating dispersion. The dispersion of U.S. Pat. No. 6,057,075 exhibits no yield point (the minimum force or shear stress required to initiate flow of a non-Newtonian dispersion).
Flexible photoreceptor belts are often fabricated by depositing layers of photoactive coatings onto long webs which are thereafter cut into sheets. Layers of such belt photoreceptors, such as charge generation layers, are often applied to belts by slot or slide coating of a dispersion.
Depending on the coating facility and the actual dispersion system utilized, different rheological properties of dispersions may be required for coating a photoreceptor. For example, while a Newtonian dispersion with no yield point may be adequate to form a uniform coating on a drum photoreceptor, a non-Newtonian dispersion with a yield point may be desirable for fast freezing-in the coated film of a dispersion with low viscosity on a flexible belt or web photoreceptor device. However, difficulties arise in utilizing non-Newtonian dispersions to coat belt or web photoreceptors due, in part, to the fact that it is not easy to uniformly mill the entire non-Newtonian dispersion and that undesirable heavy impurity particles can not be easily removed from the dispersion. Therefore, some heavy impurity particles or large particles that are greater in size than the acceptable size of the particulate additive for the given layer, may be present in the millbase. Under-milled particles and heavy impurities in dispersions utilized to form charge generation layers in photoreceptors are believed to be a primary cause of charge deficient spots (background/spots, and the like, sometimes referred to herein as CDS) in xerographic prints.
Methods which may be utilized to attempt to reduce these CDS include centrifugation and filtration of the dispersion to remove large particles/contaminants. However, as noted above, for some manufacturing processes it may be desirable to utilize non-Newtonian dispersions to form charge generation layers. For non-Newtonian dispersions, it may be extremely difficult, if not impossible, to utilize centrifugation to separate contaminants from the dispersion as the flocculated dispersion may completely separate, thereby breaking up the dispersion into its individual components.
The size of pigment particles in a charge generation layer and the distance between pigment particles within the charge generation layer may have a significant effect on ghosting performance. Ghosting is an image memory effect wherein a faint residual image appears when a new image is printed. Ideally, the distance between pigment particles within the charge generation layer should be as short as possible. For a given particle size, the distance between pigment particles, sometimes referred to herein as the particle separation distance, can be reduced by increasing the pigment to binder ratio (P/B ratio) of the dispersion. Alternatively, the particle size can be reduced at a constant P/B ratio. One drawback with these methods, however, is that they both may lead to an unstable dispersion as the P/B ratio continues to increase or the particle size is reduced below a certain level. For example, some methods for processing dispersions suitable for forming charge generation layers include mixing all components, for example solvents, binder solutions and pigments, with roughly the desired final P/B ratio; milling the dispersion to an endpoint particle size; diluting the dispersion to a given solids content; centrifuging to remove large under-milled particles and heavy impurities; and diluting the resulting dispersion to the desired final solids content for coating. While this process may enhance the stability of resulting dispersions, the achievable P/B ratio may be limited to a maximum value, depending upon the particle size, solids content, and solvent ratio. Without changing the formulation, a further increase in P/B, increase in % solids content, or a reduction of particle size while maintaining the dispersion stability may be extremely difficult, if not impossible, to obtain by milling alone.
The cost to develop different coating dispersion formulations for different layers of a photoreceptor, and the need to change dispersions for different products in the manufacturing process, greatly increases the costs to manufacture photoreceptors. Economical methods for developing these layers thus remain desirable.